Article
The spiral wound gasket is the standard gasket for raised face flanges in process plants. It handles high pressure, high temperature and thermal cycling better than a soft gasket, and it recovers some of its thickness if the joint moves. ASME B16.20 sets its dimensions and construction.
How it is built
- Windings: a V-shaped metal strip, wound in a spiral. This is what gives the gasket its springiness. Usually 304 or 316L stainless steel; alloys such as Inconel or Monel for aggressive service.
- Filler: a soft material wound between the metal strips, which does the actual sealing. Flexible graphite is the most common, suitable for a wide temperature range. PTFE is used for chemical resistance at lower temperatures. Mica-based fillers are used above the temperature limit of graphite in oxidising conditions.
- Outer (centring) ring: a solid ring, usually carbon steel, that centres the gasket inside the bolt circle and stops it being over-compressed.
- Inner ring: a solid ring on the bore side. It prevents the windings from buckling inwards, keeps the filler out of the flow, and reduces turbulence at the joint.
When is an inner ring required?
ASME B16.20 requires an inner ring for gaskets with PTFE filler, and for certain sizes and pressure classes where buckling of the windings is a known risk. Many project specifications go further and require inner rings on every spiral wound gasket. Given how small the extra cost is, fitting one is the safer default.
Colour coding
B16.20 assigns a colour code on the outer ring to identify the winding metal and a stripe to identify the filler, so a gasket can be checked on site without paperwork. The ring is also stamped with the size, class, materials and the standard. If the markings do not match the order, the gasket should not be installed.
Thickness
The common winding thickness for B16.20 gaskets is 4.5 mm (0.175 inch), compressed in service to around 3.2 to 3.3 mm. The outer ring is 3 mm (0.125 inch). The thickness matters because it affects the stud bolt length.
The five things to state on an order
- The flange size and pressure class, and the flange standard (ASME B16.5 or B16.47 Series A or B).
- The winding material.
- The filler material.
- The inner ring material, or that none is needed.
- The outer ring material.
For example: "Spiral wound gasket to ASME B16.20, 4 inch Class 300, for ASME B16.5 flange, 316L windings, flexible graphite filler, 316L inner ring, carbon steel outer ring."
See spiral wound gaskets, the gaskets and sealing range, and our guide to flange facings.
Frequently asked questions
What is a spiral wound gasket made of?
A V-shaped metal winding with a soft filler such as graphite or PTFE, usually with an outer centring ring and sometimes an inner ring.
When is an inner ring needed?
Inner rings are recommended for PTFE-filled gaskets, high classes and corrosive or vacuum service, and are often mandatory by project specifications.
Which standard covers spiral wound gaskets?
ASME B16.20 covers dimensions and marking for spiral wound gaskets for ASME B16.5 and B16.47 flanges.
Key takeaways
- Specify size, class, winding, filler, inner and outer ring materials.
- Match the gasket to the flange standard and facing.
- Use a new gasket every time the joint is opened.
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